index.html

<!doctype html>
<html lang="en">
  <head>
    <meta charset="utf-8"/>
    <title>What is my IP?</title>
  </head>
  <body>
    <h1 style="font-family: monospace; text-align: center; font-size: 10em;" id="ip">Finding IP ...</h1>
    <script>
      var RTCPeerConnection = window.RTCPeerConnection || webkitRTCPeerConnection || mozRTCPeerConnection;
      var peerConn = new RTCPeerConnection({'iceServers': [{'urls': ['stun:stun.l.google.com:19302']}]});
      var dataChannel = peerConn.createDataChannel('test');  // Needs something added for some reason
      peerConn.createOffer({}).then((desc) => peerConn.setLocalDescription(desc));
      peerConn.onicecandidate = (e) => {
        if (e.candidate == null) {
          document.getElementById("ip").innerText = /c=IN IP4 ([^\n]*)\n/.exec(peerConn.localDescription.sdp)[1];
        }
      };
    </script>
  </body>
</html>